1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for determining positions of underwater objects.
2. Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those undertaking the offshore drilling need to know where to drill in order to avoid a dry well.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure under the seafloor. While this profile does not provide an accurate location for the oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high resolution image of the structures under the seafloor is an ongoing process that requires the deployment of many seismic sensors and the recording of various seismic waves.
One method for recording the seismic waves is now discussed with regard to FIG. 1. This method is appropriate when a distance from the surface of the water to the bottom of the water is large, for example, larger than 200 m. During a seismic gathering process, a vessel 10 drags an array of seismic detectors 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other spatial arrangements than horizontally. The vessel 10 also drags a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downwards toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upwardly until it is detected by a detector 11 on the streamer 12.
However, the reflected seismic wave 24 (primary) is not only recorded by the various detectors 11 (the recorded signals are called traces) but also may reflect from the water surface 14 as the water surface acts as a mirror for the sound waves, e.g., reflectivity one. The waves reflected by the water surface are called ghosts in the art and these waves are reflected back towards the detector 11. The ghosts are also recorded by the detector 11 but with a reverse polarity and a time lag relative to the primary.
As discussed above, the recorded traces may be used to determine the structure of the sub-structure (i.e., earth structure below surface 20) and to determine the position and presence of reflectors 22. However, to be able to determine the position of reflectors 22, an accurate position of the detectors 11 is necessary.
Another method for recording seismic waves uses fixed sensors placed on the bottom of the region to be investigated as shown in FIG. 2. This method is appropriate for shallow waters, when the distance from the surface of the water to the bottom of the water is 200 m or less. FIG. 2 shows the bottom 30 of the water and a reflector 32 in the subsurface. A first vessel 34 tows a seismic source 36 with the seismic source 36 being provided below the surface 38 of the water. Detectors 40 are provided on the bottom 30 of the water. The detectors 40 are connected via cables 42 to a recording vessel 44. This technology is called ocean bottom cable (OBC). Ocean Bottom Seismometers may also be used for recording seismic waves. The Ocean Bottom Seismometer is a self contained data-acquisition system which free falls to the ocean floor and records seismic data generated by airguns and earthquakes. Similar to the method shown in FIG. 1, the positions of the detectors 40 need to be known in order to determine the position of the reflector 32.
For determining the positions of the sensors for OBC, the following techniques are common in the industry: (1) using the drop or placement coordinates of the detectors, and (2) deploying high-frequency acoustic sensors attached to the detectors and positioned independently of the seismic survey and determining the positions of the detectors based on the high-frequency acoustic sensors. The positions of the sensors may be inferred by using the first seismic source arrivals.
Because drop positions in the first technique must be recorded to assure that the actual detector locations are near the planned locations, drop positions are the cheapest and easiest to implement. In calm shallow water (such as an inland bay where the detectors may be placed on or thrust into the muddy bottom), the detector drop position can be close to the resting position. However, in deeper water or in agitated surf zones, this is unlikely due to waves, currents and drop trajectories.
The second technique, which is disclosed in U.S. Pat. No. 4,641,287, the entire disclosure of which is incorporated herein by reference, uses acoustic transponders located on a seismic cable that connect the sensors. FIG. 2 shows acoustic transponders 46 placed at various positions. The acoustic transponders are interrogated by a dedicated source boat (not shown). The acoustic pulse's frequency emitted by the dedicated source boat is in the 30 kHz to 100 kHz range, i.e., a high frequency range. Repeating the interrogation at different known locations allows the operator of the boat to triangulate and deduce the precise pinger position of the sensors 40.
However, there are not as many acoustic transponders as seismic sensors. Furthermore, the acoustic transponders are located on the cable, in-between the seismic sensors. Thus, the positions of the sensors are interpolated from acoustic pingers positions, which give approximate results.
A system described in U.S. Pat. No. 6,005,828, the entire disclosure of which is incorporated herein by reference, couples the acoustic transponders with the seismic sensors, which improves the localization of the sensors.
However, the existing technologies are not capable to exactly determine the positions of the sensors and also require the presence of acoustic transponders, which make the entire equipment complex and prone to failures. Further, if less transponders than sensors are used, the accuracy cannot be improved over a certain threshold. If each sensor is provided with a transponder, the complexity and the weight of the system increases. Accordingly, it would be desirable to provide systems and methods that provide an accurate positions of the sensors without the acoustic transponders.